Formation mechanism,development regularity and exploration implica-tion of oblique fan delta in steep slope zone of rift lacustrine basin in the early stage of rifting: a case from northern belt of Dongying sag
LI Jin1, LIU Zhen2, ZHANG Zhongmin1, LIU Jianping3, SUN Xiaofan2, LIU Huimin4, YANG Guang5, ZHU Maolin2, WANG Wenbin5, XIAO Hong2
1 Petroleum Exploration and Production Research Institute,Sinopec,Beijing 100083,China; 2 College of Geosciences,China Universety of Petroleum(Beijing),Beijing 102249,China; 3 School of Petroleum Engineering,Chongqing University of Science and Technology,Chongqing 401331,China; 4 Shengli Oilfield Company,Sinopec,Shandong Dongying 257015,China; 5 Petroleum Exploration and Production Research Institute,Shengli Oilfield Company,Sinopec,Shandong Dongying 257015,China
Abstract：As one of the most important types of oil and gas basins in the world,the rift lacustrine basin can often form glutenite sedimentary formations with a thickness of thousands of meters in the early stage of filling and evolution process,which has become an important field of oil and gas exploration. However,there are still many controversies regarding the sedimentary genesis of this extremely thick glutenite. In this study,Yanjia area in the steep slope zone of the Dongying sag,a typical rift lacustrine basin in Jiyang Depression,is selected as an example. Utilizing high-resolution 3D seismic data,core samples,logging,and well logging,we conduct a systematic analysis of the glutenite sedimentary formations in the steep slope zone during the early rifting stage. This study applies seismic stratigraphy,structural geology,sedimentology,and rheology analysis methods comprehensively. The following understandings have been obtained: (1)The gravel content in the sediments of oblique fan delta is relatively high,which is formed by traction flow and gravity flow. The average quartz content is about 40%-55%. Fan delta plain,fan delta front and pre-fan delta subfacies are developed. The seismic response characteristics include(oblique,imbricate and broom-shaped)progradation facies along the direction of sediment source,and filling facies perpendicular to the sediment source direction. (2)The oblique fan delta mainly developed in the early stage of rifting,and disappeared in the middle stage of rifting. The fan and the boundary fault are obliquely intersected at a low angle,typically not exceeding than 45°. The development scale of the fan is affected by the location of its root,and its disappearance is not isochronous. (3)A new mechanism for the formation of the large-scale oblique fan delta in the steep slope zone of the rift lacustrine basin during the early stage of rifting is proposed. Relay structures related to the segmentation characteristics of boundary faults provide transport conduits for the development of oblique fan deltas. The paleo-troughs controlled by the boundary fault activity provide new accommodation for the development of oblique fan deltas during the early stage of rifting. The fracture zone formed by the interaction between faulted blocks provide material basis for the development of oblique fan deltas,and the connection of the faulted block leads to the disappearance of the oblique fan delta and the transformation to the nearshore subaqueous fan. (4)It has been found that most of the deep glutenite reservoirs in the steep slope zone of the rift lacustrine basin belong to the oblique fan deltas. This study points out that the relay zones at all levels in the steep slope zone can be key target areas for exploration,and the well layout direction follow the oblique boundary fault. This study not only enriches the sedimentary theory of steep slope zone in rift lacustrine basin,but also provided a new idea and theoretical basis for deep petroleum exploration of the continental rift lacustrine basin in China.
LI Jin,LIU Zhen,ZHANG Zhongmin et al. Formation mechanism,development regularity and exploration implica-tion of oblique fan delta in steep slope zone of rift lacustrine basin in the early stage of rifting: a case from northern belt of Dongying sag[J]. JOPC, 2024, 26(4): 816-833.
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